The present invention relates to a paper refeeding device incorporated in a copier, printer or similar image forming apparatus for temporarily stacking paper sheets each coming out of an image forming section and carrying an image on one side thereof on a refeed tray and, then, refeeding the paper sheets to the image forming section.
A paper feeding device for use in a copier, printer or similar image forming apparatus may be implemented by an air knife and sucking means, as disclosed in Japanese Patent Laid-Open Publication No. 56-56442, for example. Specifically, an air knife blows air against one end of a stack of paper sheets loaded on a tray so as to lift the paper sheets, while sucking means urges the lowermost paper sheet on the tray against transporting means by suction to separate it from the others. The paper sheets held in such a condition are sequentially fed out from the tray, the lowermost one being first. This type of paper feeding device is attracting much attention because it achieves more rapid paper feed than a paper feeding device of the type using a pick-up roller which has to be moved up and down every time a paper sheet is fed. The paper feeding device using an air knife selects the highest air velocity when a great number of paper sheets exist on the tray, while reducing the air velocity stepwise as the number of paper sheets decreases. Even when only a small number of paper sheets remain on the tray, the air knife of the prior art device constantly blows air at a velocity high enough to lift the paper sheets overlying the lowermost paper sheet away from the latter.
Assume that the air knife type paper feeding device is implemented as a paper refeeding device incorporated in a copier for executing two-sided copy mode and combination copy mode operations. Then, the air knife constantly blows air against the end of the paper stack on a refeed tray at a velocity high enough to lift the paper sheets. Hence, if the number of paper sheets is less than ten, for example, and the intermediate portion of the paper sheets is curled upward away from the tray or the paper sheets are bent due to low elasticity, the air blowing against the end of the paper stack aggravates the curl. When the transporting means transports the paper sheet in the above condition by a predetermined distance in a predetermined direction and then brings it to a halt, the edge of the paper sheet will be located at a position deviated from an expected position where the edge would be located if the the curl were not aggravated. Further, assume that all the paper sheets for producing copies are stacked on the tray and then refed one by one. In this case, air is also constantly blown against the end of the paper stack at a velocity high enough to lift the latter. Since the weight of the entire stack sequentially decreases with the number of paper sheets on the tray, the stack bodily waves up and down and thereby increases the period of time necessary for the lowermost paper sheet to be urged against the transporting means by the suction exerted by the sucking means.
When an attempt is made to enhance rapid paper transport by reducing the interval at which the transporting means transports the paper sheets, the transporting means will begin driving the paper sheet before the latter is pressed thereagainst by the suction, again failing to feed out the paper sheet. Thus, the transport speed available with the prior art device is limited despite the use of air.